Diagnostic imaging techniques, such as magnetic resonance imaging (MRI), X-ray, nuclear radiopharmaceutical imaging, ultraviolet-visible-infrared light imaging, and ultrasound, have been used in medical diagnosis for a number of years. Contrast media additionally have been used to improve or increase the resolution of the image or to provide specific diagnostic information.
To be effective, the contrast media must interfere with the wavelength of electromagnetic radiation used in the imaging technique, alter the physical properties of tissue to yield an altered signal, or, as in the case of radiopharmaceuticals, provide the source of radiation itself. MRI and optical methods are unique among imaging modalities in that they yield complex signals that are sensitive to the chemical environment. While the signal from X-ray or radionuclide agents remains the same whether agents are free in plasma, bound to proteins or other targets, or trapped inside bone, certain contrast agents for MRI and optical imaging will have different signal characteristics in differing physiological environments. It is important that the contrast agent be sufficiently sensitive and present at high enough concentration so that signal changes can be observed.
Complexes between gadolinium or other paramagnetic ions and organic ligands are widely used to enhance and improve MRI contrast. Gadolinium complexes increase contrast by increasing the nuclear magnetic relaxation rates of protons found in the water molecules that are accessible to the contrast agents during MRI (Caravan, P., et al., R. B. Chem. Rev. 99, 2293 (1999)). The relaxation rate of the protons in these water molecules increases relative to protons in other water molecules that are not accessible to the contrast agent. This change in relaxation rate leads to improved contrast of the images. In addition, this increase in relaxivity within a specific population of water molecule protons can result in an ability to collect more image data in a given amount of time. This in turn results in an improved signal to noise ratio.
Imaging may also be performed using light, in which case an optical dye is chosen to provide signal. In particular, light in the 600-1300 nm (visible to near-infrared) range passes relatively easily through biological tissues and can be used for imaging purposes. The light that is transmitted through, or scattered by, reflected, or re-emitted (fluorescence), is detected and an image generated. Changes in the absorbance, reflectance, or fluorescence characteristics of a dye, including an increase or decrease in the number of absorbance peaks or a change in their wavelength maxima, may occur upon binding to a biological target, thus providing additional tissue contrast. In some situations, for example the diagnosis of disease close to the body surface, UV or visible light may also be used.
A need persists for contrast agents that can deliver sufficient concentrations of the imaging moiety to the target to improve the sensitivity of the imaging process as well as contrast agents that have a sufficient half-life in vivo.